1. Field of the Invention
The present invention relates to meandering (snaking) of a photo-sensitive belt (including a transfer belt) on which Yellow (Y), Magenta (M), Cyan (C), and Black (BK) intermediate images are overlapped before transferred to a recording medium in a laser color printer.
2. Description of the Related Art
The laser color printer may be such that four color developing units are switched from one to another by a mechanical drive unit, so as to be brought into contact with a photosensitive drum. More specifically, the four color toner images are formed while the photosensitive drum makes four turns. In contrast to this, as shown in FIG. 7, there is a printer where four of photosensitive drums 501 are aligned for the four colors. That is, for each of the colors, an electrostatic latent image is formed by an exposure unit 502, and then a toner image is formed on the photosensitive drum 501 using the respective development units 503. The toner image is transferred onto a recording medium 505 by operating a transfer roller 506 matched with the feed of the recording medium 505 traveling on the paper feed path 504. The image is finally fixed by a thermal roller 507 and a pressurizing roller 508. This method is called tandem type.
As shown in FIG. 8, the tandem type may be such that along the photosensitive belt, there are arranged exposure units 602 such as laser and development units 603 of four colors, so that the electrostatic latent image formed on the photosensitive belt 601 is covered with toner. The obtained toner image is transferred to the recording medium 605 by operating the transfer roller 606 traveling along a paper feed path 604. The image is finally fixed by a thermal roller 607 and a pressurizing roller 608.
There is also a composite type printer of FIG. 7 and FIG. 8. That is, the toner image on the photosensitive drum 501 is not transferred directly to the recording medium 505 but transferred to an intermediate medium, i.e., a transfer belt.
The most important thing about a color printer performance is that the toner images of four colors should be obtained with a high resolution, i.e., with allowance of 1/2 of the dot size or below. In the aforementioned photosensitive drum method, the paper feed accuracy affects this resolution, and in the photosensitive belt method, the photosensitive belt traveling accuracy affects the resolution. Moreover, in the composite type transfer belt, the transfer belt traveling accuracy affects the resolution.
Since the present invention intends to increase accuracy in the photosensitive belt type color printer, hereinafter, explanation will be given on conventional problems in the belt type.
As shown in FIG. 9, in the photosensitive belt type, a photosensitive belt 701 is driven by a drive roller 702 and supported by a plurality of auxiliary rollers 703. Here, it is necessary to prevent skew, i.e., belt width direction deviation caused by the difference in length of both edges of the photosensitive belt 701 and deviation in mechanical parallelism of the auxiliary rollers 703. (It should be noted that transfer belt has identical drive configuration as the photosensitive belt and its explanation will be omitted.)
A steering roller 704 is supported in such a manner that the angle can be changed by a steering arm 705. The steering roller is provided with a cam 708 and a return spring 709 so that its inclination can be changed around the designed mechanical parallelism as a center by a pulse motor 707 via a reduction gear 706.
A belt edge sensor 710 provided so as to sandwich the photosensitive belt monitors whether the photosensitive belt 701 is traveling along predetermined position of the rollers. That is, if the belt edge sensor has detected that the photosensitive belt 701 deviates in one direction, the steering roller 704 is inclined so that the photosensitive belt 701 is moved in the opposite direction. Thus, the belt is rotated along a predetermined path.
However, such a configuration has problems as follows. Since the belt edge sensor 710 and the steering roller 704 are provided apart from each other, there is a time delay between them. When the belt edge sensor 710 decides that the deviation has become zero, the steering roller 704 has already inclined in the opposite direction. Since this occurs repeatedly, there arises a meander of the photosensitive belt around its rotation center. The photosensitive belt is stable around the drive roller 702 and deviates most at the steering roller.
Here, even if the meander arises in the photosensitive belt 701, if only one image (for example one A4 sheet) is formed within one meander cycle, the timing can be obtained by the belt edge and there is not danger of dot position shift between the four colors. That is, performance as the printer is allowable.
Actually, however, there is a case when a plurality of images are formed for each of the colors on the photosensitive belt 701. In this case, there is a danger of meander direction change between the colors.
Accordingly, when viewed in the main scan direction, the image is matched at the start point but the end portion is shifted by the meander.
In the aforementioned printer apparatus using the steering mechanism and the photosensitive belt, a positional shift up to 100 to 200 micrometers occurs. The belt is dedicated for transfer function so as to eliminate the limit of the belt holding mechanism. For example, an abutment guide is formed on the roller or the belt is provided with a guide protrusion, so that the belt is fed normally using a method other than the steering mechanism.
However, in order to apply this method to a photosensitive belt, there arise a problem that it is difficult to produce a seamless photosensitive belt, and a seam is indispensable, which results in a stepped portion.
In order to reduce the stepped portion, production costs should be significantly increased. Furthermore, because of the substrate material of the photosensitive belt, if the photosensitive belt is regulated by an abutment portion, the auxiliary roller cannot have an appropriate sliding.
The aforementioned problems of the conventional configuration using the steering roller can be summed up as follows.
The belt edge state detected by the belt edge sensor is used by the steering roller with a time delay. That is, the steering is controlled after detecting that the belt edge position is at a reference position, when the steering roller control is further performed. As a result, the belt inevitably meanders.